1. Field of the Invention
This invention relates to a developing device for use in an image forming apparatus such as a copier, a printer or a facsimile apparatus.
2. Description of the Related Art
There is a well known method of bearing a dry developer on the surface of a developer bearing member, conveying and supplying the developer to the vicinity of the surface of an image bearing member on which an electrostatic latent image is formed, developing the electrostatic latent image with the developer while applying an alternating electric field between the developer bearing member and the image bearing member, and visualizing the electrostatic latent image.
Generally, a development sleeve is often used as the developer bearing member and therefore, the developer bearing member will hereinafter be referred to as the xe2x80x9cdevelopment sleevexe2x80x9d, and a photosensitive drum is often used as the image bearing member and therefore, the image bearing member will hereinafter be referred to as the xe2x80x9cphotosensitive drumxe2x80x9d.
The above-described developing method is classified roughly into four kinds of development, i.e., monocomponent noncontact development in which for a nonmagnetic toner, the development sleeve is coated with the toner by a blade or the like and for a magnetic toner, the development sleeve is coated with the toner by a magnetic force, the toner is carried to the photosensitive drum by the rotation of the development sleeve, and the electrostatic latent image on the photosensitive drum is developed with the toner being in noncontact with the photosensitive drum; monocomponent contact development in which the electrostatic latent image is developed with the toner being in contact with the photosensitive drum; two-component noncontact development in which for a developer consisting of a magnetic carrier mixed with a nonmagnetic toner is borne on the development sleeve by a magnetic force, the developer is carried to the photosensitive drum by the rotation of the development sleeve, and the electrostatic latent image is developed with the developer being in noncontact with the photosensitive drum; and two-component contact development in which the electrostatic latent image is developed with the developer being in contact with the photosensitive drum. From the two viewpoints of higher quality and higher stabilization of the image, two-component contact development is often used.
A toner image formed on the photosensitive drum by the development of the electrostatic latent image is transferred onto a transferring material such as paper, whereafter it is fixed by heat, pressure, etc., whereby a final copy image is obtained. On the other hand, the photosensitive drum after the transfer of the toner image has an adhering contaminant such as any untransferred toner residual on its surface removed by a cleaner, and thereafter is repeatedly used for image formation.
In recent years, in image forming apparatuses using a two-component developing device, development for still a higher quality of image, a longer life (higher stabilization) and downsizing has been put forward. To achieve the longer life of the developing device, it is important to present the deterioration of the toner and carrier due to the compression of the developer to thereby lengthen the life of the developer.
The place in the container (developing container) of the developing device where the developer is compressed is a developer layer thickness regulating portion, and in the construction of an ordinary developing device, the developer layer thickness regulating pole of a magnet in the development sleeve is located upstream of a developer layer thickness regulating blade with respect to the direction of rotation of the development sleeve, and in this area, the developer attracted to the layer thickness regulating pole is compressed between the development sleeve and the developing container.
To weaken the compression of the developer, it is effective to weaken the force with which the developer layer thickness regulating pole attracts the developer to the development sleeve (magnetic attraction acting in a direction perpendicular to the surface of the development sleeve).
As a method therefore, mention may be made of making the magnetization of the magnetic carrier in the developer small (the direction in which this magnetization of the carrier is decreased is a direction advantageous to a higher quality image in that the force rubbing against the developed toner image on the photosensitive drum in a developing portion becomes weak), on building up such a magnetic pattern that it is difficult for magnetic lines of force from the developer layer thickness regulating pole to go around to adjacent magnetic poles and the magnetic lines of force go out perpendicularly as far as possible from the surface of the development sleeve.
As one of the methods using the latter magnetic pattern, there has been proposed a developing method using one of the repulsive magnetic poles of the magnet in the development sleeve as the developer layer thickness regulating pole. When magnetic poles of the same polarity are adjacent to each other and form a repulsive magnetic field, the magnetic lines of force of each of those magnetic poles goes out perpendicularly to the surface of the development sleeve, and the rate of change of the magnetic flux density in the direction perpendicular to the surface of the development sleeve is small. As a result, the force attracting the developer to the development sleeve becomes small, and the compression of the developer weakens. Also, as a matter of course, the amount of developer itself collected at the developer layer thickness regulating pole becomes small and from that point as well, the compression of the developer weakens.
In such a case, if there is adopted a method of installing the developer layer thickness regulating pole and the regulating blade in the lower portion of the developing device with respect to the direction of gravity, magnetically attracting and drawing the developer onto the development sleeve, and stripping the developer after development in the portion above the layer thickness regulating pole with respect to the direction of gravity, the amount of developer itself collected in the layer thickness regulating portion can be made smaller and the compression of the developer can be made lighter in pressure, and it becomes easy to downsize the developing device by the simplification of the construction of the developing device.
Relative to the developing device in which the developer layer thickness regulating pole and regulating blade are installed in the lower portion of the developing device with respect to the direction of gravity, the exposed portion of the photosensitive drum is usually located on the upper portion of the photosensitive drum and therefore, the direction of rotation of the development sleeve and the direction of rotation of the photosensitive drum often become counter to each other.
On the other hand, for the downsizing of the developing device as a single body, it is the only method to downsize such parts as the development sleeve and a developer conveying screw constituting the developing device. Recently, regarding the downsizing of the main body of a copier, there has also been proposed a construction in which a drum cleaner is detached from the main body of the copier and cleaning simultaneous with developing is effected (a cleanerless system), but again in that case, the aim at the downsizing of the main body of the copier cannot be achieved unless each of the steps of charging, exposing, developing, transferring and fixing is made compact.
As described above, the development sleeve of the developing device is made small in diameter and one of the repulsive poles of the magnet in the development sleeve is used as the developer layer thickness regulating pole (cut pole), whereby it becomes possible to construct a developing apparatus achieving a longer life.
However, when one of the repulsive poles is used as the developer layer thickness regulating pole, uneven image density in the form of the pitch of the developer conveying screw (uneven screw pitch) has occurred to the trailing end of a black solid image.
There are two types of uneven screw pitch, i.e., one of an uneven T/D ratio and one of an uneven M/S ratio. The T/D ratio is the ratio of the weight of the toner to the total weight of the toner and carrier of the developer, i.e., the toner density of the developer. The M/S ratio is a ratio of an amount (weight) of developer on the development sleeve per unit area of the development sleeve.
A description will first be made of the occurrence of the uneven screw pitch by the first-mentioned uneven T/D ratio. When one of the repulsive poles is used as the developer layer thickness regulating pole, the developer is liable to have the characteristics that (a) the developer after development stripped by the repulsive magnetic pole is liable to move to the developer layer thickness regulating pole, but (b) the amount of developer collected at the developer layer thickness regulating pole is small and the compression of the developer is weak.
Therefore, the developer coats the development sleeve in a state in which the stripped developer (the toner is being consumed by development) and the developer newly supplied to the development sleeve by the rotation of the conveying screw do not completely mix with each other. As the result, an uneven T/D ratio in the form of screw pitch occurs to the developer on the development sleeve, and the uneven screw pitch thereby occurs to the trailing end of the developed image.
A description will now be made of the occurrence of the uneven screw pitch by the second-mentioned uneven M/S ratio. Because of the above-described characteristic (b), when the conveying screw is located near the developer magnetically attracted to the developer layer thickness regulating pole, uneven compression occurs to the developer collected at the developer layer thickness regulating pole by the rotation of the screw. As a result, in the lengthwise direction of the development sleeve, an uneven coat in the form of screw pitch, i.e., an uneven M/S ratio, is formed in the developer on the development sleeve, and uneven screw pitch attributable to the uneven M/S ratio occurs at the trailing end of the developed image.
The phenomenon of the uneven screw pitch of the image by the above-described uneven T/D ratio and uneven M/S ratio becomes more liable to occur when the diameter of the development sleeve is small, in addition to the use of one of the repulsive magnetic poles as the developer layer thickness regulating pole. This is because when the diameter of the development sleeve is small, the magnetic force becomes small and the distance from the stripping pole which is the other of the repulsive magnetic poles as the developer layer thickness regulating pole becomes short and therefore the developer after development becomes liable to move from the stripping pole to the developer layer thickness regulating pole.
Such uneven screw pitch is also liable to occur when the magnetization of the carrier is made small to lengthen the life of the developer. This is because when the magnetization of the carrier is small, the developer becomes dull to a magnetic field and the developer after development is not stripped by the stripping pole, but becomes liable to move to the developer layer thickness regulating pole.
It is an object of the present invention to provide a developing device which can obtain good images free of uneven density.
Further objects of the present invention will become apparent from the following detailed description when read with reference to the accompanying drawings.